The invention relates to thermal barrier coatings and thermal barrier coating systems for high temperature applications, such as gas turbine assemblies.
The design of modern gas turbines is driven by the demand for higher turbine efficiency. It is widely recognized that turbine efficiency may be increased by operating the turbine at higher temperatures. Typically, various techniques are used to apply bondcoats and thermal barrier coatings to airfoils and combustion engine components of the turbine, such as transition pieces and combustion liners, to assure a satisfactory life span at these higher temperatures.
Usually, the thermal barrier coatings are configured to tolerate strain in the underlying component without detaching from the component. The thermal barrier coatings are usually made of ceramic materials, which have relatively lower inherent ductility than their underlying metallic components; hence, various microstructural features are typically incorporated into the thermal barrier coating to provide the thermal barrier coating with improved strain tolerance. For instance, the thermal barrier coatings deposited by plasma spray processes typically incorporate significant porosity, vertical microcracks, or both, as a means to enhance the ability of the thermal barrier coating to tolerate strain. By way of example, thermal barrier coatings deposited by vapor processes, such as physical vapor deposition (PVD), typically are fabricated under conditions that encourage nucleation and growth of discrete, tightly packed, columnar grains, which provides a compliant microstructure with a relatively high degree of strain tolerance.
Although PVD processes provide coatings with suitable strain tolerance on relatively small components as compared with plasma spray processes. However, compared to the plasma spray processes the PVD processes require expensive set-up including a vacuum chamber and supporting equipment. On the other hand, conventional thermal spray processes tend to produce coatings with lower strain tolerance and substrate adhesion than PVD processes, and generally require ancillary surface preparation processes, such as grit blasting and deposition of rough bondcoats, to provide adequate adhesion to the underlying component.
The bondcoats are typically used to promote adhesion of the thermal barrier coating layer to the underlying component and inhibit oxidation of the underlying component during high temperature exposure of the component. Typically, bondcoats having aluminide coatings are used in thermal barrier coating systems to provide oxidation resistance to the substrate and to enhance adhesion of the thermal barrier coatings. In order to have adequate adhesion, plasma sprayed thermal barrier coatings are typically deposited on bondcoats with rough surfaces such as overlay MCrAlY bondcoats. Relatively smoother bondcoats, such as those formed by vapor phase aluminide (VPA), often are not considered suitable candidates for depositing thermal barrier coatings deposited by plasma spray methods.
Therefore, there is a need for thermal barrier coatings that exhibit high strain tolerance, high adhesion, and reduced need for surface preparation processes that can be applied via comparatively inexpensive and scalable processes such as plasma spray processes.